1. Field of the Invention
The present invention relates to a power semiconductor device. More particularly, the present invention relates to the structure of a power semiconductor device in which a bipolar transistor and a diode are formed `antiparallel` within the same semiconductor chip, the above stated power semiconductor device being applied to an inverter circuit and the like. As the term is used herein `antiparallel` means a parallel connection in such a way that the current conducting directions of the respective devices are opposite each other.
2. Description of the Prior Art
FIG. 1 is an electric circuit diagram of an ordinary conventional inverter circuit and FIG. 2 is a waveform diagram for explaining the operation of the circuit in FIG. 1.
In FIG. 1, an emitter of a transistor 1 is connected with a collector of a transistor 2 and an emitter of a transistor 3 is connected with a collector of a transistor 4. A DC power source 9 is connected between the collector of the transistor 1 and the emitter of the transistor 2 as well as between the collector of the transistor 3 and the emitter of the transistor 4. A capacitor 11 is connected to the DC power source 9 in parallel. The transistors 1, 2, 3 and 4 are connected in antiparallel to the diodes 5, 6, 7 and 8, respectively, so that return current may flow therethrough respectively.
In the inverter circuit having the above described structure, voltage is applied to the load 10 as shown typically in FIG. 22 and in a period `a` in which load voltage is applied in the positive direction. The transistors 1 and 4 conduct and transistors 2 and 3 are inactive. The output control of the load 10 can be determined by the ratio of the conducting period and the interrupting period `b` of the transistors and by the state of conduction of either group of transistors 1 and 4 or transistors 2 and 3. During the interrupting period b, the diodes 5-8 are respectively connected in antiparallel to the operating transistors 1 to 4. This can be attained by interrupting the contact of the conducting transistors 1 and 4. However, if the load includes an inductance component, counter electromotive force is applied the cause to flow of electric current having the same current value as that of the current flowing in the load 10 in the period b and the period a. Due to this counter electromotive force, electric current flows in the diodes 6 and 7 and by return of this current to the DC power source 9, the current value in the load 10 becomes constant and the voltage applied to the load 10 is reversed. In the period a' and the period b' in FIG. 2, electric current flows in the diodes 5 and 8.
In the inverter circuit in FIG. 1, the voltage of the DC power source is generally approximately 2/3 of the rated voltage of the transistors 1, 2, 3 and 4. As a result, if a short circuit occurs in the inverter circuit, a large current flows in the transistors 1, 2, 3 and 4 in the state in which high voltage is applied thereto, resulting in distribution of transistors 1, 2, 3 and 4 in an extremely short period. If the transistors 1, 2, 3 and 4 are destroyed, the extremities of the chips of the transistors are generally melted, bringing about a short-circuited state between the associated collectors and emitters. In the event of short circuit, a protection circuit generally functions in the inverter circuit so that operation of the DC power source is interrupted. However, if the time required for interrupting the contact of the DC power source 9 is long, or if the capacitor 11 connected to the DC power source 9 has a large capacity, the lead wires connecting the emitter electrodes of the chips of the transistors 1, 2, 3 and 4 are easily melted and it sometimes happens that an open state occurs between the associated bases and emitters. After melting of the emitter lead wires, electric current applied from the DC power source 9 flows from the collectors to the bases of the transistors. Since this electric current is relatively large and the collector-base contact is sometimes broken, voltage from the DC power source 9 is caused to be applied directly to the bases and the base-drive circuit is easily broken, causing sometimes firing, which is a serious problem.
FIG. 3 is a circuit diagram of an apparatus by which a lead wire melting test is applied to a bipolar transistor employed in the inverter circuit of FIG. 1. FIG. 4 is a graph showing the results of the lead wire melting test.
Referring to FIG. 3, available DC power source 13 is connected to a resistor 14, between a collector and an emitter of a transistor 12 subjected to the test, and a capacitor 15 is connected in parallel with a series circuit comprising the resistor 14 and the variable DC power source 13.
FIG. 4 shows the results of examination of the phenomenon of the lead wires of the above described transistor in the testing circuit shown in FIG. 3. In FIG. 4, the horizontal axis represents the total cross-sectional area of the lead wires and the vertical axis represents energy (CVV/2) stored in the capacitor 15. The character a in FIG. 4 indicates resistance of an aluminum wire to melting, the character b indicates resistance of a silver lead wire to melting and the character c indicates resistance of an aluminum wire to melting in case where the thickness of a chip aluminum electrode is increased twice as much. The character d will be specifically described later.
Generally, resistance of a transistor to breakage is increased if the number of lead wires is increased or the thickness thereof is made large, but the resistance is decreased if the capacity of the capacitor 15 is increased. If the thickness of the emitter electrode is increased, little change will be caused in the resistance. As a result, it can be said that for a transistor to which aluminum lead wires are attached by supersonic bonding, it is extremely difficult to resist melting in case of an ordinary capacity of a capacitor. By contrast, a transistor to which silver lead wires are attached by soldering can have resistance several times as much as that of a transistor using aluminum lead wires.
However, a transistor including lead wires attached by soldering has numerous disadvantages, such as low productivity, low reliability of a chip or an assembly structure, or poor chip performance characteristics due to the fact that a fine pattern cannot be formed. Accordingly, at present, most power transistors are manufactured by bonding of aluminum wires.
FIG. 5 is a perspective view of an example of aluminum wire bonding in a conventional transistor. FIG. 6 is a sectional view taken along the line VI--VI shown in FIG. 5. FIG. 7 is a sectional view of a conventional transistor in which lead wires are attached by soldering.
Referring to FIGS. 5 and 6, a transistor chip 16 is structured by forming a collector low impurity concentration region 18 on a collector high impurity concentration region 17 and by forming thereon a base region 19 and a emitter region 20 successively. In the collector high impurity concentration region 17, a collector electrode 23 is formed; in the base region 19, a base electrode 22 is formed; and in the emitter region 20, an emitter electrode 21 is formed. An emitter lead wire 24 is connected to the emitter electrode 21 and a base lead wire 25 is connected to the base electrode. In FIG. 6, the arrows indicate distribution of electric current flowing in the emitter. FIG. 7 is a state in which an emitter lead wire 24 is attached to the emitter electrode 21 by solder 241.
As a result of the test using the melting test apparatus shown in FIG. 3, it was determined that the resistance to melting of a soldered aluminum lead wire and that of a soldered silver lead wire cannot be explained only by the difference of conductible current capacity of the aluminum lead wire and that of the silver lead wire. This difference should be considered as follows. The portion where the path for emitter current in case of aluminum wire bonding is smallest, causing melting most often is considered to be the peripheral portion of the wire bonding portion in the emitter aluminum electrode 21 on the chip 16. The reasons for this are that the thickness of the aluminum electrode is 4 .mu.m to 6 .mu.m while the diameter of an aluminum wire is generally 300 .mu.m to 400 .mu.m and that lead wire melting resistance depends on the length of the peripheral portion of the aluminum wire bonding portion because of the fundamental structure in which, as shown by the arrows in FIG. 6, electric current in the transistor chip 16 hardly flows under the emitter bonding pad and the electric current flowing into the finger portion of the emitter region 20 is collected to the lead wire 24 through the aluminum electrode 21.
As for resistance to melting in the peripheral portion of the wire bonding portion, a noticeably advantageous structure is adopted in a lead wire soldering system as compared with an aluminum wire system since a solder layer of several tens of .mu.m to several hundreds of .mu.m is formed around a lead wire attached by soldering. The above described consideration is supported by the below described results of an experiment.
FIG. 8 is a perspective view of an example in practical use of a conventional transistor for an inverter; FIG. 9 is a diagram of an equivalent circuit; and FIG. 10 is a sectional view of a return current diode.
The transistor for the inverter in FIG. 8 comprises a collector electrode 23 on which a base electrode 22' and an emitter electrode 21' are formed through an insulating plate 29 and through an insulating plate 29, 30, respectively, and a monolithic Darlington transistor chip 26 and a return current diode chip 28 are formed. On the base electrode 22, a speed-up diode chip 27 is formed and on the monolithic Darlington transistor chip 26, a resistor portion 31 is formed.
When a complex device having a structure in which the above described return current diode chip 28 as in FIG. 8 is disposed adjacent the transistor chip 26 is subjected to a test in the testing circuit of FIG. 3, melting resistance in an aluminum wire system was in some cases equal or higher than that in a lead wire soldering system, as shown in FIG. 4'd. Thus, if an aluminum wire system exhibits almost the same melting resistance as that of a lead wire soldering system, there is a distinctive characteristic that the return current diode 28 is broken. The breakage of the return current diode 28 can be regarded as being caused when the transistor chip 26 is violently broken since the return current diode 28 is in close contact with the transistor chip 26. In such a case, the emitter wire of the transistor chip 26 is melted, while the wire of the return current diode 28 is hardly melted, large current flowing in the broken return current diode 28.
The return current diode 28 comprises, as shown in FIG. 10, a cathode low impurity concentration region 33 formed on a cathode high impurity concentration region 32 and an anode region 34 formed thereon. A cathode electrode 36 is formed on the cathode high impurity concentration region 32 and an anode electrode 35 is formed on the anode region 34. An end of an anode lead wire 37 is connected to the anode electrode 35 and the other end of the anode lead wire 37 is connected to an emitter electrode 21'. When the return current diode 28 is broken, electric current flows in the direction shown by the arrows in FIG. 10. As is clear from FIG. 10, the electric current flowing at the time of breakage of the return current diode 28 becomes largest immediately under the wire bonding and it is considered that the reason why the wire of the diode hardly melts is that the contact area functions effectively while in case of a transistor, melting is caused dependently on the peripheral length of the contact of the wire bonding portion.